Isotope Exchange for Gas-Phase Acetic Acid and Ethanol at Aqueous Interfaces: A Study of Surface Reactions

Abstract

Isotope exchange for deuterated gas-phase acetic acid and ethanol in contact with water (H2O) droplets was studied using a droplet train apparatus. In these experiments, the gas-phase species interacts with liquid droplets and the loss of the species is monitored. The loss of the species may be due to the entry of the molecules into the bulk or to a reaction of the species at the gas−liquid interface, in this case isotope exchange. Studies were conducted as a function of pH in the range 0−14, droplet temperature in the range 291−263 K and gas−liquid interaction time in the range 2−15 ms. For deuterated acetic acid the isotope exchange probability with water molecules at the interface is near unity. On the other hand, isotope exchange probability for ethanol with surface water molecules at pH 7 is much smaller, ranging from 0.033 at 263 K to 0.051 at 291 K. Ethanol isotope exchange is both acid and base catalyzed. The exchange probability therefore increases both toward low and high pH and levels off to a ...